The present invention relates to a process for the recovery of lead from lead scraps, such as discarded batteries.
The most common form of scrap lead is that contained in discarded batteries. This scrap has usually been treated pyrometallurgically, by roasting the lead sulfate present in the scrap into oxides and by reducing the obtained oxide into metallic lead, at a temperature of approx. 800.degree. C in a shaft furnace with the aid of carbon and carbon monoxide. In the said process the sulfur dioxide reduced from the lead sulfate plus relatively large amounts of metallic lead are removed along with the flue gases. For the above reasons the said flue gases have to be treated in gas washers, with which most of the sulfur dioxide and metallic lead can be recovered but a complete purification cannot be achieved. This recovery of the lead compounds removed along with the flue gases is described in German Pat. No. 1,937,323, in which chlorine-bearing lead gases are washed by means of a sulfate solution. The washing and filtration of the flue gases is extremely expensive no matter which basic process is used for treating the lead pyrometallurgically, and all of the lead can never be recovered from these gases.
Pyrometallurgical processes have a further disadvantage in that the separation of lead from its natural impurities is difficult; in molten state it requires the use of various oxidizing chemical and the skimming of impurities in slag form from the surface of molten lead, the slag carrying a substantial part of the lead along with it. Moreover, the purity of the lead thus obtained, at best 99.7 %, is not sufficient for all purposes.
For this reason attempts have been made to find a different solution to the problem of treating used batteries and other similar scraps. British Pat. No. 1,385,957 discloses how lead is leached by means of an ammonium sulfate solution, from which the lead is recovered by causing the obtained solution to be absorbed into porous carbon and by burning the carbon, whereby the free lead is recovered in molten state. Although relatively pure lead is obtained by a simple method in this process, the disadvantages of the pyrometallurgical process have not been eliminated.
Other leaching processes are also known. In U.S. Pat. No. 3,689,235 the lead oxide and lead sulfate present in batteries is leached by means of a soda solution, from which it is dried and reduced into metallic lead as above with the aid of carbon.
The use of chlorides for the leaching of the said lead salts is described in British Pat. No. 456,807, in which the lead is leached to form a chloride and precipitated thereafter as a pure sulfate, and the chemicals used are regenerated. The obtained pure lead sulfate can obviously be reduced into pure lead by pyrometallurgical means, but the environmental hazards have not been eliminated. Hydrochloric acid is used for the leaching of lead-bearing metal salts and metal in the process described in Danish Pat. No. 131,309, in which, however, the lead is precipitated out of the solution by means of sulfides of other metals, and thus a pure lead salt is again obtained.
The use of chlorides for the leaching of lead sulfate and the removal of the sulfate from the solution by precipitation are described in German Pat. No. 72,804. However, a sulfate-free lead chloride, which is the prerequisite for economical electrolysis, is not obtained by this method. In the electrolysis, sulfate causes wearing of the graphite electrodes, increases the power requirement, and causes SO.sub.2 formation.
German Pat. No. 343,345 and Swedish Pat. No. 49,441 describe methods of leaching lead from lead ores by chloride and sulfuric acid solutions. In both publications, however, the lead is reduced into metallic form by pyrometallurgical means.
German Pat. No. 429,667 describes as an improvement a two-stage leach for leaching the lead in chloride form; in the first stage a hot concentrated chloride solution is used for converting the lead compounds into lead chloride, and in the second stage of large amount of a colder chloride solution is used for causing the lead chloride to pass into the solution.
This process has the disadvantage that the amount of solution required for dissolving lead chloride at a low temperature is large and the precipitation of lead chloride becomes complicated. Furthermore, the concentrated solution of the first stage is diluted by the dilute solution of the second stage, and so the method cannot be applied to a process with a closed circulation of solution.
In the process according to the present invention all the above substantial disadvantages have been eliminated, and an especially pure lead can be produced by it.
Furthermore, it has been observed that, compared with conventional processes, the manufacturing costs are considerably lower when using the process according to the invention.